Mrs Coleman believes the pollution is from the Cat Hill housing development on the former Middlesex University Campus in Cockfosters, where L&Q plans to build 231 homes.

Andrew Dismore, London Assembly Member for Barnet and Camden, also sent a letter to the chief executive of the Environment Agency Dr Paul Leinster, demanding he uses his powers to halt construction on the site while Pymmes Brook is tested for contaminants.

But a spokeswoman for the Environment Agency has said there is no evidence to prove the deposits are from the Cat Hill development.

In a statement, the Environment Agency said: “We are aware of the concerns raised about potential contaminants entering the watercourse in Oak Hill Park in North London.

“Our Environment officers have visited the site and found no evidence to confirm that the sandy deposits had come from the Cat Hill demolition site, but we will continue to monitor the situation.”

A spokeswoman from L&Q said: "We’re pleased that the Environment Agency was able to visit the site and confirm that there is no contamination entering the water as a result of our activities. We’re committed to managing the ponds, and creating a better environment for local wildlife in the long term."